Sheet element processing machines typically comprise an introduction station where sheets (usually paper sheets and/or cardboard sheets) are provided to a feed table. The feed table is situated upstream of a cutting machine or a platen press. The sheet element is advanced against one or more front tabs by first means such as endless belts or rollers, then delivered by second means against one or more lateral positioning tabs before the front edge of the sheet element is gripped by a series of grippers mounted on a gripper bar arranged on a chain system.
Such a device is used for precise lateral positioning of sheet elements which have already undergone one or more printing operations, wherein the subsequent operation may be either a stamping process, for example hot foil stamping in a platen press, or a cutting and waste discharge operation in such a press. This subsequent operation must be performed in strict accordance with the preceding printing.
Lateral positioning devices are today used for jogging the sheets. They comprise firstly a lower roller driven in rotation and arranged transversely to the direction of movement of a sheet, close to a lateral tab situated on the left side of the table as viewed also in the direction of movement of the sheet, which is normally known as the operators side. However there are two lateral positioning devices on a machine, one on the operator side and one on the opposite operator side. The operator is free to work with one or the other depending on the requirements of the layout of the final product. These devices then comprise an upper roller, vertically above the lower roller, and mounted at the end of an arm which is in the top position at rest. This arm is lowered regularly on arrival of a sheet element against the frontal tabs, such that the upper roller grips the sheet element against the lower motorized roller which, by traction on the sheet element, causes a correction movement of the sheet element as far as the lateral tab.
There are different approaches in the prior art for detecting an unwanted condition in which sheets are not supplied individually but in a superimposed condition.
Document EP 0669274 describes a lateral positioning device for a sheet element on a feed table, with elements for holding the sheet element (by traction or thrust) with extended surfaces. The aim is thus to avoid damaging the grip surfaces of the sheet element. The singularity of the sheet element engaging in the lateral positioning device is here verified by a complementary device, situated at the inlet to the positioning device and comprising an upper roller and a lower roller situated in the same vertical plane. The spacing of these rollers is set to the value of the thickness of a single sheet element.
Document JP 3426850 describes a positioning device wherein the sheet element is moved laterally in one or the other transverse direction by means of a guidance device comprising two pairs of upper and lower rollers situated in the same vertical plane, each mounted on different lateral sides of the device. Each pair of rollers may be disengaged and the direction of rotation of the rollers may be reversed in order to allow driving of the sheet element in the required direction to correct its positioning. However, this method of gripping tends to mark sheet elements of the corrugated cardboard type, which are more susceptible to crushing than is flat cardboard.
Document JPS 6047751U describes a device with a pivoting lever arm carrying, at its end directed towards the sheet element, a freely rotating roller situated above a drive wheel which is continuously rotated by an endless screw, in order to take the sheet element assembly and deliver it by traction against the lateral stop. The roller of the pivoting lever may be moved away or retracted to switch from the pull mode of moving the sheet element to the push mode of moving the sheet element.
Document JPH 0430203 (JPS62147642) describes a positioning device in which the sheet element is moved laterally in the one or the other transverse direction by means of a guidance device comprising a pair of upper and lower rollers situated in the same vertical plane. The upper roller is freely rotatably mounted, and the direction of rotation of the lower roller may be reversed to allow driving of the sheet element in the required direction in order to correct its positioning. The same guidance device is present on each lateral side of the positioning station.